Anti-tumor activity expressed in vitro by adherent mouse peritoneal leukocytes is being analyzed with regard to two main questions: (1) The nature of cellular subpopulations mediating activity in different systems, with special attention to the role of macrophages vs. nonphagocytic adherent cells (a cell more akin to B lymphocytes); and (2) The biochemical basis of tumor cell cytotoxicity, with special attention to the role of oxidative metabolism by macrophages after sequential activation and triggering. The role of cell surface receptors in triggering will be analyzed.